Isoprene is a reactive diene that can undergo a variety of reactions including thermal dimerization. The first report of the thermal dimerization of isoprene was recorded in the late 1800's. In the absence of air or peroxides, only cyclic dimers are formed in the thermal dimerization of isoprene. Six cyclic dimers are formed.
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They are 1,5-dimethyl-5-vinyl-1-cyclohexene (I), 1-methyl-4-isopropenyl-1-cyclohexene, also known as limonene (II), 1,4-dimethyl-4-vinyl-1-cyclohexene (III), 1-methyl-5-isopropenyl-1-cyclohexene, also known as diprene (IV), 2,5-dimethyl-1,5-cyclooctadiene (V) and 1,5-dimethyl-1,5-cyclooctadiene (VI). The relative amounts of the six dimers are dependent on the reaction conditions. When the thermal dimerization is conducted at temperatures ranging from about 100.degree.-190.degree. C., dimers 1-methyl-4-isopropenyl-1-cyclohexene and 1-methyl-5-isopropenyl-1-cyclohexene (diprene) constitute 57% to 80% of the dimer product.
One major problem with the thermal dimerization of isoprene at high temperatures is that much of the isoprene is lost to the formation of polymer. A number of inhibitors have been tried to reduce polymer formation with various degrees of success. For example, G. Solomon, et al, Rubber Chemistry and Technology, 22, 956 (1949) disclose that isoprene dimerized in the presence of 10,000 ppm of picric acid at 200.degree. C. produced 32% by weight polymer. J. Binder, et al., J. Poly. Sci., 38, 229 (1959) disclose isoprene dimerized in the presence of 1000 ppm of hydroquinone at 227.degree. C. produced 10.8 weight percent polymer. German DE No. 2,421,820 discloses that a blend of 4000 ppm sodium nitrosylpentacyanoferrate/50 ppm t-butylcatechol has been used to reduce isoprene polymerization to as low as 0.5% at 150.degree. C. during dimerization.
Limonene, also known as 1-methyl-4-isopropenyl-1-cyclohexene occurs in various oils, and particularly in the oils of lemon, orange, caraway, dill and bergamot. Although limonene can be produced from the thermal dimerization of isoprene, it is generally isolated from the peels of citrus fruits, so the price can fluctuate dramatically depending on how large the citrus crop is. Limonene is commonly used as a solvent, wetting and dispersing agent or as a component in high softening, water white tackifying resins. Since there are a number of commercial applications of limonene, but there being no consistent and economical way to produce it, there is a significant need for a cheap and dependable process to provide limonene in good yields.